Lubricants for driveline power transmitting devices (such as gears or transmissions), especially axle fluids, automatic transmission fluids (ATFs), and manual transmission fluids (MTFs)), present highly challenging technological problems and solutions for satisfying the multiple and often conflicting lubricating requirements, whilst providing durability and cleanliness. One of the important parameters influencing durability is the effectiveness of phosphorus antiwear or extreme pressure additives at providing devices with appropriate protection under various conditions of load and speed. However, many of the phosphorus antiwear or extreme pressure additives contain sulphur. Due to increasing environmental concerns, the presence of sulphur in antiwear or extreme pressure additives is becoming less desirable. In addition, many lubricating compositions containing antiwear or extreme pressure additives evolve volatile sulphur species resulting in an odour and potential detriment to health and the environment.
A lubricating composition having the correct balance of phosphorus antiwear or extreme pressure additives provides driveline power transmitting devices with prolonged life and efficiency with controlled deposit formation and oxidation stability. However, many of the antiwear or extreme pressure additives employed have limited oxidative stability, form deposits, or increase corrosion. In addition, many phosphorus antiwear or extreme pressure additives typically contain sulphur, which results in an odorous lubricating composition containing the phosphorus antiwear or extreme pressure additives. A number of references disclosing antiwear chemistry are discussed below.
U.S. Pat. No. 5,338,470 discloses alkylated citric acid derivatives obtained as a reaction product of citric acid and an alkyl alcohol or amine. The alkylated citric acid derivative is effective as an antiwear agent and friction modifier.
U.S. Pat. No. 4,237,022 discloses tartrimides useful as additives in lubricants and fuels for effective reduction in squeal and friction as well as improvement in fuel economy.
U.S. Pat. No. 4,952,328 discloses lubricating oil compositions for internal combustion engines, comprising (A) oil of lubricating viscosity, (B) a carboxylic derivative produced by reacting a succinic acylating agent with certain amines, and (C) a basic alkali metal salt of sulphonic or carboxylic acid.
U.S. Pat. No. 4,326,972 discloses lubricant compositions for improving fuel economy of internal combustion engines. The composition includes a specific sulphurised composition (based on an ester of a carboxylic acid) and a basic alkali metal sulphonate.
U.S. Patent Application 60/862,534 (PCT/US07/082,057) discloses malonate esters suitable as antiwear agents.
International Publication WO 2005/087904 discloses lubricants containing hydroxy carboxylic acid and hydroxy polycarboxylic acid esters in combination with phosphorus-containing additives. The phosphorus-containing additives include zinc dihydrocarbyldithiophosphates and/or neutral phosphorus compounds, such as trilauryl phosphate or triphenylphosphorothionate. The lubricants are useful in engine lubricants.
International Publication WO 2006/044411 discloses a low-sulphur, low-phosphorus, low-ash lubricant composition containing a tartrate ester, or amide having 1 to 150 carbon atoms per ester or amide group. The lubricant composition is suitable for lubricating an internal combustion engine.